In Visual Studio, most of the time we need to keep track of the classes or forms you are navigating through and we need them to be highlighted in the Solution Explorer.

This option could be switched off sometimes, therefore the picture below shows you how to switch this option on again.

Therefore, to track the Active Items just tick on the 'Track Active Item In Solution Explorer' checkbox and click OK.

1) How to Register User Controls and Custom Controls in Web.config

2) Creating and Using User Control Libraries 

3) Register Custom Controls In Web.config 

To add controls to the Visual Studio IDE from an assembly, just drag this assembly or DLL from Windows Explorer to

VS IDE toolbox and you will get all the controls showing on the Toolbox.